Waste water drain heat exchanger preheating cold water supply to a thermostatic control valve

ABSTRACT

A bath waste assembly is that unit that occupies the lowest point in a vessel or tray used for bathing and drains away water to a sewage system, often in concert with an overflow waste drain. The horizontal portion of the waste water assembly terminates in a P-trap that separates the assembly from the rest of the drain and sewer apparatus. The horizontal pipe running from the bath drain hole to the fitting leading to the P-trap can become nearly as hot as the hot water originally entering the vessel, particularly when the bath is being used as a shower vessel. Replacement of this pipe with a copper waste water pipe wrapped with narrower copper pipe through which cold supply water runs, absorbing heat on its way to a thermostatic control valve, allows the amount of hot water balancing this cold water to be decreased automatically since the temperature of the cold water has been increased. Since the heat transfer avoids the hot water supply entirely, various benefits are involved including complete independence from the water heating technology, reduction of heat loss and pressure loss, better heat transfer, and a decreased risk of cross-contamination.

REFERENCES CITED U.S. Patent Documents

U.S. Pat. No. 4,619,311* Vasile

U.S. Pat. No. 5,736,059* MacKelvie

U.S. Pat. No. 6,722,421B2* MacKelvie

BACKGROUND

A building has several different water flows. One is cold water to end use faucets, a second is cold water to a water heating device, a third is hot water to end use faucets, and a fourth is used drain water. The result is complex flow times, flow rates, pressures balances, flow volumes, and flow temperatures.

To effectively recover heat from drain water, heat exchange must occur with the drain water in a manner that makes instantaneous use of the heat from that drain water; otherwise, the heat exchange benefit will be lost through cooling. Further, that part of the drain water pipe system off which heat exchange is effected ought to be as close as possible to the drain hole draining hot water. This is to avoid two phenomena: the mixing of the water with other cold waste water, including toilet flush water, from other fixtures, and the accumulation of organic slime on the interior of that drain water pipe, effectively insulating the drain water from the outer copper wall.

These limitations contraindicates the use of the great vertical drain waste stack in the house, for the following reason. Toilet and other organic residues building up on the inside of this stack act as the aforementioned insulating blanket of organic slime, and other cooler sources of water mix with the hot waste water from that bath. Another strong incentive to avoid the great drain waste stack is that replacement of the cast-iron or plastic pipe with a massive copper insert as per Vasile (U.S. Pat. No. 6,722,421 B2) becomes an expensive task given the size and cost of copper pipe involved and the risks inherent in modifying the most critical drainpipe in the house.

Existing heat exchange patents focus on the removal of heat from drain water to heat cold water leading to a water heating device such as a tankless water heating device. This results in a loss of heat over the length of the vector pipe, this also results in water pressure loss, and this also results in the necessity of timing hot water use with hot water preheating. Although easier for instant water heaters than for hot water tanks, quantifying losses for consumers when costing out their return on investment becomes an inexact and highly variable exercise involving pipe lengths, average water use lag times, and other variables.

The Invention described in this application answers these limitations through redirection of the pre-heated cold water not to a water-heating device, but instead to the cold water portion of a thermostatic control valve located by necessity within close proximity of the horizontal drain waste. An increase in the temperature of the cold water supply allows the control valve to mix in less hot water in achieving the target temperature.

DESCRIPTION OF THE DIAGRAM

FIG. 1 describes the Invention. The cold water supply No. 1 passes into a coil of narrow copper pipe No. 2 wrapped tightly around a horizontal drain waste water pipe No. 3 running between the drain hole No. 7 and the P-trap No. 8, and then exits back through a port No. 4 leading to the thermostatic control valve. The cold water supply, heated through its exchange with warm waste water, after being fed to the thermostatic control valve No. 10, is mixed with the quantity of hot water necessary to achieve the target water temperature as selected by the user. The resultant tempered water passes through a single supply pipe No. 5 and issues into the bath No. 10 via any variety of spouts or shower heads here exemplified by No. 6, and let pass into the drain hole No. 7 by an open stopper device to start the heat exchange cycle again, less entropy. The vertical portion of the waste overflow No. 9 is not involved in heat exchange.

DETAILED DESCRIPTION OF THE INVENTION

By way of review, in a building, in a household cold water is always under pressure, and flows into the water heating device when a hot water faucet is opened. In modem, safe, baths, this cold water is directed to the bath after passing through a thermostatic control valve that mixes the cold supply water and hot supply water to maintain the selected temperature independent of pressure variance (e.g. the toilet flushing during another water user's shower).

The thermostatic control valve is a newer technology, and this application describes how this control enables a new way for water preheated through heat exchange with warm drain waste water to short circuit the water heating device entirely. To whit, cold supply water is preheated in a copper coil winding around the horizontal copper pipe, and that pre-heated water is piped not a great distance away to a water heating device, but instead to directly feed the cold water supply port of a thermostatic control valve. Less hot water is needed to balance the cold water, and the heat is directly recovered all within approximately a one foot radius of the heat recovery from the drain waste horizontal pipe member.

This is the top of the evolution of patented heat recovery devices heating cold water with heat from drain water to save energy and money. An examination of key patents in this evolution follows below.

U.S. Pat. No. 4,619,311 to Vasile, describes a drain water heat recovery system comprising a vertical copper drainage heat exchanger whose exterior is wrapped with a copper walled void for cold supply water to be preheated through heat exchange. This type of tube-on-tube heat exchanger has been long marketed by various North American manufacturers. Since the two exchangers are in direct physical contact, beneficial heat transfer occurs when both drain water and cold water are flowing simultaneously, as when showering. Losses in heat occur because the water heating device, the shower or bath, and the heat exchanger inserted into the vertical great drain waste stack are almost never all in proximity.

Moreover, when hot water tanks are used provide hot water the time delay between filling and drainage of that tank creates a lag between the cold water preheated by the heat exchanger and the actual use of pre-heated hot water, further reducing useful energy savings. Further, cold drain water from other sources will cool the water in the outer coil. These drawbacks severely limit energy savings.

U.S. Pat. No. 5,736,059 to MacKelvie does describe a drain water heat recovery system with no-loss heat storage. However, for low volume hot water users, such as in homes, the system tends to be too large, and with its numerous components, too expensive. Further, its installation is essentially limited to vertical drainpipes unless mechanical pumping is added.

MacKelvie tries again in U.S. Pat. No. 6,722,421 to elaborate on Vasile's original device by greatly increasing the complexity and cost of the vertical heat exchanger, yet the device in its entirety is still limited by the heat loss constraints inherent in supply of the preheated cold water supply to a water heating device. In contrast, the objective of the present Invention is to provide a low cost, easy to install drain water heat exchanger that is directly linked to water outflow, ‘washing out’ the variable of the water-heating device and the length of piping leading to it. 

1. Given a plumbing system for a building includes a supply of a first cold water liquid and a supply of a second liquid compromising waste water draining from a vessel, the Invention comprises an apparatus to transfer heat between said second liquid to the said first liquid via a double-walled interface, said apparatus including and connected directly to the cold water port of thermostatic control valve. If waste water drains vertically through a vessel drain hole that constitutes the lowest point of the vessel, and then through an elbow that redirects by 90 degrees that flow to a horizontal pipe member, the means by which heat exchange is effected is the coiling of the first liquid supply pipe around the horizontal drain waste water pipe member. The horizontal drain waste water pipe is made of copper and optionally invaginated in the same coil pattern to receive and anchor the coil of narrower copper leading from the cold water supply source ultimately to the cold water port of the thermostatic control valve.
 2. The improvement of claim 1 wherein said plumbing system includes a means for heating said first cold water liquid, said apparatus being connected such that said cold water supply passes through first the heat exchanger prior to entering a thermostatic valve for tempering supply water, directly reducing the need for hot water supply to the same thermostatic control valve in order to achieve the water temperature selected by the user on the thermostatic temperature control independent of the water heating device heating the hot water supply and the distance of pipe connected the drain to the water heating device and back to the hot water port of the thermostatic control valve. This reduces heat and water pressure loss relative to previous drain waste heat exhangers.
 3. The improvement of claim 1 wherein drain waste liquid in horizontal pipe member is sent to exchange its heat content to the incoming cold supply water avoids the build up of organic slime within the drain waste pipe over time. Indeed, if the horizontal pipe member is optionally invaginated in a coil pattern to receive and anchor the coil of narrow copper pipe wrapped round it, the resulting turbulence will decrease organic slime formation and increase the heat exchange coefficient. This is in contrast to locating the heat exchanger in the great drain waste stack, where toilet, kitchen sink and other sources of organic matter will eventually deposit an insulating blanket of organic slime that will decrease the efficiency of the heat exchange.
 4. The improvement of claim 1 wherein the decrease in the mass of copper used for a given flow of warm waste water in the Invention, in contrast to heat exchangers used for vertical drain waste heat exchange, reducing the latent heat sink of bringing the copper of the heat exchanger up to the heat of the waste water, lessening the efficiency of the heat exchanger.
 5. The improvement of claim 1 wherein the location of the double walled heat exchanger is moved from the vertical great drain waste stack of a house to the horizontal drain pipe beneath the bath or shower vessel drain. Although double walled copper is a safe way to keep fresh supply water from cross-contamination with drain water, failure may happen. Cross-contamination of bath or shower water with bath or shower water is undesirable, but far more safe than cross-contamination with the toilet water passing down through the waste water stack. The present Invention is safer in terms of cross-contamination than the vertical heat exchangers mentioned in the patents reviewed. 